And Barney is still chuckling over this discussion. I appreciate the faith
that some people put in me by expressing their support of my utterances
just because I am me, but in fact Barney was only half right (well, maybe
75% right). And I appreciate the fact that Dan went out of his way to
verify that part for which he was skeptical.
At 10:59 PM 8/14/98 EDT, Dan Masters <DANMAS@aol.com> wrote:
>In a response to one of my rambling posts, Barney Gaylord wrote:
>
>> .... Older single coil systems pass the spark from the coil (via the
distributor) through the spark plug to ground, and the high voltage circuit
is completed by the body of the coil also being grounded.
>>
>> With the waste spark system the body of the coil is not grounded, but
the high voltage side of the coil completes the circuit by being connected
to spark plugs, both of which are grounded. The spark goes from the coil
through one plug to ground and then from ground through the other plug back
to the coil to complete the circuit. ....
>> ....
>> Just one of our modern day curiosities, but it does show that modern
day designers have determined that the polarity of the spark plug is pretty
irrelevant.
Whereupon Dan goes on to pile on the praises after having verified my
description of the circuitry of the waste spark ignition coil. But alas,
only a few people have caught the fact that the single spark ignition coil
does not have the high voltage side grounded to the case. My statement was
a simplification that served well to illustrate the point that spark plug
polarity is not too significant, but the time has come to set the record
straight about how a single spark ignition coil actually works.
So here I sit looking at the wiring diagram in my MGA factory Workshop
Manual, and that diagram clearly shows that none of the circuitry in the
ignition coil is connected to the case of the coil. Both the primary and
the secondary windings have one end connected to the (C.B.) terminal which
goes to the contact breaker points to be grounded when the points are
closed. The other end of the primary winding is powered by the ignition
switch, while the other end of the secondary winding is the high tension
lead that goes (via the distributor cap and rotor) to a spark plug with an
air gap (and ground on the other side of the gap).
Now when the switch is on and the points are closed, the primary side of
the coil is energized and a magnetic field builds up in the core of the
coil. At the same time, the secondary side of the coil has no current
flowing because it is apparently an open circuit with the air gaps in the
dizzy and the spark plug.
When the contact points open, the current in the coil primary winding stops
and the magnetic field in the core suddenly collapses, generating a HUGE
voltage surge in the secondary winding. This voltage is so high that it
forces a current to flow across the air gaps in the dizzy and at the spark
plug. There is a substantial amount of energy dissipated here in a very
short period of time (the spark).
While this big spark is happening, the contact breaker points are open
circuit, so the remaining circuit takes an interesting path. Starting with
the battery hot terminal it goes through the ignition switch, then through
the primary winding of the coil, then through the secondary winding of the
coil, and on through the dizzy and spark plug to ground. Also there is a
capacitor ("condenser") wired across the contact points to ground. Now
it's time for the brain tickler and quiz.
1.) Why does the high energy spark not fry the ignition switch and all the
other devices that draw power from it?
2.) What is the function of the capacitor in this circuit?
3.) Will the system generate a spark without the capacitor being connected?
4.) Will the system generate a spark with the capacitor being shorted out
internally?
And now two questions for extra credit:
5.) Exactly how and why does the coil generate such a high voltage in the
secondary winding when the magnetic field collapses?
6.) Approximately how much current is flowing in the wire from the ignition
switch to the coil when the spark is happening (within one order of
magnitude)?
Barney Gaylord
1958 MGA with an attitude (and dual points with one capacitor)
P.S.
After all this is settled, there may be a discussion on radio interference,
carbon plug wires, and how capacitors may be used in other places to
surppress electrical noise. (This is a trick).
BG
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